A ring communications network is made up of nodes that are connected in tandem by a unidirectional communications path. Each node receives transmissions from the adjacent upstream node, and if the communication is destined for a downstream node, the communication is re-transmitted to the adjacent downstream node. Otherwise, each node transmits its own communications to the adjacent downstream node.
A drawback of such a network is that a break in the ring would prevent any node upstream of the break from communicating with any node downstream of the break. Similarly, the complete failure of a node would have the same effect as a break in the ring.
Many designs have been proposed to minimize these difficulties. The most common approach is to provide a second communications ring parallel to the first. In that case, a fault in one ring could be bypassed by transferring communications to the second ring. Alternatively, if the second ring transmitted in the opposite direction as the first, a break in both rings between two adjacent nodes could be remedied by the nodes on either side of the break looping back communications received on one ring onto the other ring. Such a system is described in McNeilly et al, U.S. Pat. No. 3,652,798.
The main problem with such approaches is that the equipment required to detect and locate a fault, and then appropriately reconnect transmitters and receivers with the alternate ring, is complicated and costly.
An approach that alleviates some of these difficulties is described in Lau, U.S. Pat. No. 4,835,763, which is assigned to the same assignee as the present invention. In Lau, a subrate multiplexed signal is utilized for ring communications. Each node has the capability of demultiplexing the main signal into its constituent subrates (channels), and channels destined for that node (local channels) are sent to receiving equipment within the node, while channels destined for downstream nodes (through channels) are multiplexed with originating local channels, and the resultant high level signal is transmitted to the adjacent downstream node. This process is simultaneously performed using identical equipment in the node for a second ring transmitting in the opposite direction. If a node detects a fault in an incoming line, an error signal is placed on all of the channels following the demultiplexing. The receiving equipment in each node includes a selector which monitors the communications arriving on each local channel from both rings. If an error signal is detected on a local channel, the selector selects the communication from the associated channel of the other ring to send to the receiver.
In this way, a break in both rings between two adjacent nodes will not cause a failure in the system, and no complicated fault locating and switching equipment is required to continue service. Similarly, the complete failure of a node will not destroy communications among the remaining nodes.
It should be noted that unlike prior survivable ring arrangements which maintain their ring characteristics following a fault, the arrangement of Lau ceases functioning as a ring if the ring is broken. However, communications among the nodes is maintained following such a break. For this reason, the arrangement of Lau has been called a hybrid ring, since it normally operates as a ring, but does not operate as a ring following a break in the ring or the loss of a node.
In Lau, an arrangement for interconnecting two autonomous but interrelated rings is disclosed. In this arrangement, simultaneous breaks in both rings can be compensated without loss of communications between any two nodes. The arrangement employs a master node from each ring and two unidirectional paths connected between the master nodes to couple the two rings. Even though the rings are protected from breaks in each ring, an outage in either master node or a unidirectional path breaks the communication path from one interrelated ring to the other.
From the aspect of the capacity of rings to carry multiplexed signals, it must be pointed out that because communication among different nodes are multiplexed to only one signal, traffic-carrying capacity (bandwidth) of this signal can be readily exhausted. When this happens, increasing ring capacity is costly and complicated. Links between all adjacent nodes as well as all nodes have to operate at the same rate, which has to be chosen to accommodate the largest anticipated combined communications requirement among all nodes. This is costly. Another problem with rings is interconnection of multiple rings. Synchronization of ring operation is complicated, especially under failure condition and when multiple rings are interconnected.